1. Field of the Invention
This invention relates to a method and apparatus for the forming of substantially complete enclosures from thermoplastic sheet material by vacuum molding.
2. Description of the Prior Art
Vacuum forming is a well known and commonly employed method for the forming of hollow articles from thermoplastic sheet material. The method is commonly employed for the fabrication of flat and irregularly shaped objects of relatively uniform wall thicknesses. A recent development in the field of vacuum forming is double forming wherein the molding is performed in vertically spaced apart mold halves that are mounted on moveable platens of a hydraulic or pneumatic press. The thermoplastic sheets are heated to molding temperatures, placed in the press and are drawn into conforming contact with the intaglio patterns on the die faces of the mold halves by evacuation of pressure therefrom. The mold halves are forced together to fuse the separate thermoplastic sheets in a seam about the periphery of the molded article and, to this end, the die faces have opposed lands which are pressed together to effect the fusion of the sheets.
The press, as thus employed, is quite large and requires a basement for containing the actuation mechanism for movement of the lower platen of the press. Additionally, the lands of the opposed die faces are wide because of the difficulty in precisely aligning two moving mold halves. As a result, the pressures which can be applied on the thermoplastic sheet material are generally inadequate to effect any substantial extrusion of the sheet material from between the lands of the mold halves and, consequently, the excess sheet material about the periphery of the molded product must be cut therefrom in a separate finishing operation.
The thermoplastic sheets are commonly moved into molding positions and contact with the die faces of the mold halves by supporting them on upper and lower, open frameworks which are provided with transfer means for effecting horizontal movement into and out of the molding station and vertical transport means for positioning the sheets at each station, e.g., for lowering the thermoplastic sheets into contact with the die faces of the mold halves. Desirably, the open frameworks are maintained in as close proximity to each other as possible to minimize the overall height of the press as well as to achieve the most efficient heating. Care must be taken, however, to avoid contacting the heated thermoplastic sheets with each other or with elements of the molding apparatus other than the die faces. Accordingly, caution must be taken to insure that the open frameworks remain stationary while the sheets are suspended between heating elements at the heating station or while they are being transported to the molding station. The use of fluid pressure actuators for effecting vertical relative movement of the open frameworks introduces a potential problem of creeping of the open frameworks caused by leaks of the pressure fluid in the actuator mechanism.